As written before it is possible to copy Legends between Sheets with Dynamo. An even more convenient way to do exactly the same is a tool in pyRevit called: Copy/Update Selected Viewports to Selected Sheets. Read the explanation about this tool here:
pyRevit is a library of IronPython scripts and contains a set of tools which can be very useful. The installer can be found here: eirannejad.github.io
More information about pyRevit can be found on github.com/eirannejad/pyRevit.